Fencepost Problem
A mathematician answers 'five posts' for a 60-foot fence with posts every 12 feet, leaving a broken fence; the classic off-by-one error.
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Cartoon by Ben Orlin, Math with Bad Drawings. https://cartoons.mathwithbaddrawings.com/2017-12-01-fencepost-problem/ (CC BY-NC 4.0)Transcript
A: "I want to build a 60-foot fence, with posts every 12 feet."
B: "Easy. Five posts."
[Fence drawn with a gap where a section is missing]
A: "Aren't you supposed to be a mathematician?"
B: "Hey, it's not my fault your trick question ripped a hole in the fabric of number!"